Hydrogen storage material

ABSTRACT

An unactivated, poorly activatable hydrogen storage component and an activated hydrogen storage component are mixed to prepare a hydrogen storage material. When the hydrogen storage material is activated, the poorly activatable hydrogen storage component is converted to a hydrogen storable state in a remarkably short time. The poorly activatable hydrogen storage component may be a V—Cr—Ti hydrogen storage alloy having a body-centered cubic (BCC) crystal structure. The activated hydrogen storage component preferably is MgH x  (0.1≦x≦2) doped with a nanoparticle of at least one atom selected from the group of Ni, Fe, Ti, Mn, and V.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a hydrogen storage material, which canstore hydrogen when activated.

2. Description of the Related Art

Fuel-cell cars, which use fuel cells as a power source, have recentlybeen attracting much attention from the viewpoint of environmentalprotection. Fuel-cell cars do not emit greenhouse gases such as NO_(X),SO_(X), CO₂, and hydrocarbon gases, and discharge only H₂O, which isgenerated by a reaction between hydrogen and oxygen that are containedwithin the fuel gas supplied to an anode and the oxidant gas supplied toa cathode in the fuel cell.

In general, the fuel gas and the oxidant gas are hydrogen gas and air,respectively. Therefore, a fuel-cell car includes a vessel therein forstoring hydrogen.

When a larger amount of hydrogen can be stored in the hydrogen storagevessel, the fuel cell can be operated for a longer period, and thus thefuel-cell car can be driven a longer distance. From this viewpoint,various methods for increasing the hydrogen storage capacity of thehydrogen storage vessel have been studied. In one of these methods, ahydrogen storage material such as a hydrogen storage alloy is placedinside the hydrogen storage vessel.

The hydrogen storage material is capable of storing hydrogen at a volumethat is larger than the material's own volume. Thus, by using such ahydrogen storage material, the hydrogen storage capacity of the hydrogenstorage vessel can be increased. Further, hydrogen is reversibly storedwithin the hydrogen storage material, so that the required amount ofhydrogen can be released from the material in order to operate the fuelcell.

The surface of the hydrogen storage material (particularly the hydrogenstorage alloy) is covered with an oxide layer initially, and thematerial cannot store hydrogen in this state. Thus, the hydrogen storagematerial is subjected to an activation treatment in order to reduce andremove the oxide layer at a predetermined hydrogen pressure andtemperature. The hydrogen storage material is made capable of reversiblystoring and releasing hydrogen as a result of the activation treatment.

However, it is difficult to activate a hydrogen storage material havinga body-centered cubic (BCC) crystal structure, such as a V—Ti—Crhydrogen storage alloy. For example, such a material is activated byrepeating evacuation to 10⁻⁴ torr at 500° C. and pressurization to ahydrogen pressure of 50 atm four times (see Japanese Laid-Open PatentPublication No. 10-110225), or by repeatedly evacuating, maintaining thehydrogen storage material at 400° C. at a hydrogen pressure of 8 MPa forone hour, and then cooling to room temperature three times.

In the above method, the hydrogen storage material may be activatedafter introducing the material into the hydrogen storage vessel. In thiscase, the hydrogen pressure and temperature required for activation mustbe controlled within an allowable pressure and temperature range for thehydrogen storage vessel. When a component of the hydrogen storagevessel, such as a liner or a sealant, is composed of a resin, the vesselhas an upper allowable pressure limit of 10 MPa and an upper allowabletemperature limit of 100° C. Activation of a V—Ti—Cr hydrogen storagealloy requires a long time, i.e., about 75 hours, under such pressureand temperature conditions. Thus, disadvantageously, it takes severaltens of hours to activate the hydrogen storage material at such arelatively low pressure and temperature.

A hydrogen storage alloy, which has a mixed phase of a BCC alloy phaseand a Laves phase, thus enabling the hydrogen storage alloy to be easilyactivated, is described in Japanese Laid-Open Patent Publication No.10-245653. In paragraph [0008] of Japanese Laid-Open Patent PublicationNo. 10-245653, it is conjectured that the Laves phase is easilyactivated, hydrogenated, and pulverized, so that fractures arepropagated to the poorly activatable BCC alloy phase. Then, the BCCalloy phase, which is not poisoned with air, is exposed at the fracturesurface, and the BCC alloy phase becomes hydrogenated and pulverizedfrom the exposed surface, whereby activation of the alloy isaccelerated.

In the production of a BCC hydrogen storage alloy, the mixed phase ofthe BCC alloy phase and the Laves phase can be formed by adding a smallamount of Zr during a step of melting a starting material powder. Thus,a complicated process for controlling the Zr amount, etc., is requiredwhen forming the mixed phase.

Further, Zr has a relatively large atomic weight, and therebydeteriorates the hydrogen storage capacity per unit weight of thehydrogen storage alloy.

SUMMARY OF THE INVENTION

A general object of the present invention is to provide a hydrogenstorage material, which can be activated in a short period of time, evenunder low temperature and low pressure conditions.

A principal object of the present invention is to provide a hydrogenstorage material that can be activated without complicated processes.

Another object of the present invention is to provide a hydrogen storagematerial having a large hydrogen storage capacity per unit weight.

According to an aspect of the present invention, there is provided ahydrogen storage material comprising a poorly activatable hydrogenstorage component and an activated hydrogen storage component, wherein10 hours or more are required to activate the poorly activatablehydrogen storage component at a hydrogen pressure of 10 MPa or less at atemperature of 100° C. or lower.

In the present invention, the activated hydrogen storage component ismixed with the poorly activatable hydrogen storage component. The poorlyactivatable hydrogen storage component, after being mixed with theactivated hydrogen storage component, can be activated in a remarkablyshort period of time, even under low temperature and low pressureconditions. Thus, compared to a case of activating the poorlyactivatable hydrogen storage component on its own, the time required foractivating the poorly activatable hydrogen storage component issignificantly shorter when mixed with the activated hydrogen storagecomponent. In short, according to the present invention, the poorlyactivatable hydrogen storage component can be activated remarkablyeasily.

Since the poorly activatable hydrogen storage component can be activatedunder low temperature and low pressure conditions, the hydrogen storagematerial can be activated in a short time even though it is contained ina hydrogen storage vessel. Thus, even in cases where a substancecontained in the hydrogen storage material is likely to be readilydeactivated by air, the hydrogen storage material can be activated in ashort time, while preventing deactivation of the substance by using thehydrogen storage vessel.

Further, in the present invention, an additional element is not addedduring production of the hydrogen storage material, so that acomplicated process for controlling the element amount, etc., is notrequired. In addition, the hydrogen storage capacity per unit weight ofthe hydrogen storage alloy does not become deteriorated by such anelement.

The reason that activation of the poorly activatable hydrogen storagecomponent is accelerated due to mixing thereof with the activatedhydrogen storage component is presumed to be because an active hydrogenatom stored in the activated hydrogen storage component is transferredover to an oxide layer formed on the surface of the poorly activatablehydrogen storage component, and thus the oxide layer is reduced by thepresence of the hydrogen atom.

The poorly activatable hydrogen storage component is not particularlylimited. Preferred examples thereof include hydrogen storage alloyshaving body-centered cubic (BCC) crystal structures. Thus, in thepresent invention, a substance considered to be resistant to activationcan be activated remarkably easily.

Preferred examples of the activated hydrogen storage component includeMgH_(x) (0.1≦x≦2) doped with a nanoparticle of at least one atomselected from the group of Ni, Fe, Ti, Mn, and V. Such an MgH_(x)component does not become significantly deactivated in air, andtherefore the MgH_(x) component can be easily handled. For example, theMgH_(x) component can be mixed with the poorly activatable hydrogenstorage component in air.

The above and other objects, features, and advantages of the presentinvention will become more apparent from the following description whentaken in conjunction with the accompanying drawings in which a preferredembodiment of the present invention is shown by way of illustrativeexample.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing elapsed time-stored hydrogen amount relationsof hydrogen storage materials according to an example of the presentinvention, compared with a comparative hydrogen storage material; and

FIG. 2 is a graph showing elapsed time-stored hydrogen amount relationsof hydrogen storage materials according to another example of thepresent invention, compared with another comparative hydrogen storagematerial.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of the hydrogen storage material of the presentinvention will be described in detail below with reference to theaccompanying drawings.

The hydrogen storage material according to the present embodimentcomprises a mixture of a poorly activatable hydrogen storage componentand an activated hydrogen storage component, wherein the mixture (thehydrogen storage material) is contained in a hydrogen storage vessel.

Activation of the poorly activatable hydrogen storage component requiresa long time, i.e., 10 hours or more, within allowable hydrogen pressureand temperature ranges of the hydrogen storage vessel. Morespecifically, the poorly activatable hydrogen storage component is suchthat 10 hours or more are required to activate the component at ahydrogen pressure of 10 MPa or less and at a temperature of 100° C. orlower. Preferred examples of such poorly activatable hydrogen storagecomponents include BCC hydrogen storage alloys, specifically, V—Cr—Ti,V—Cr—Al, V—Cr—Mo, V—Ti—Mo, V—W, V—Cr—Ti—Al, and V—Cr—Mo—Al hydrogenstorage alloys.

Such a BCC hydrogen storage alloy cannot be activated easily. However,once the BCC hydrogen storage alloy has been activated, it can store alarge amount of hydrogen therein, so that the hydrogen storage capacityper unit weight of the hydrogen storage material increases.

Preferred examples of the poorly activatable hydrogen storage componentsfurther include alloys having a high hydrogen storage equilibriumpressure (a gauge pressure of 1 MPa or more) at room temperature,specifically AB2 alloys (such as Ti—Zr—Fe—Cr—Ni alloys, Ti—Fe—Cr—Mnalloys, and Ti—Fe—Cr—Cu alloys) and AB5 alloys (such as La—Ce—Ni—Mnalloys and La—Ce—Ni—Fe alloys).

The activated hydrogen storage component is not particularly limited,and may be any substance as long as it is activated beforehand. Forexample, the activated hydrogen storage component may be prepared byactivating the above described poorly activatable hydrogen storagecomponent, or by activating LaNi₅, TiCr₂, or the like, all of which canbe activated relatively easily.

Alternatively, the activated hydrogen storage component may be MgH_(x)(0.1≦x≦2) doped with a nanoparticle of at least one atom selected fromthe group of Ni, Fe, Ti, Mn, and V. The term “nanoparticle” is definedas a fine particle having an average particle diameter of 10 nm or less.

Such an MgH_(x) component has a hydrogenated surface, and therefore theMgH_(x) component is not significantly oxidized in air. In other words,the MgH_(x) component is hardly deactivated, even in air. Thus, theMgH_(x) component can be mixed together with the poorly activatablehydrogen storage component in air, thus enabling the hydrogen storagematerial of the present embodiment to be produced easily. Further, thehydrogen storage material can store hydrogen slowly but steadily, evenat a relatively low temperature such as room temperature.

The nanoparticle-doped MgH_(x) component may be prepared by mixing Mgparticles and nanoparticles, followed by mechanically grinding themixture under an increased hydrogen pressure.

Before mixing of the activated hydrogen storage component with thepoorly activatable hydrogen storage component is carried out, hydrogenmay either be stored or not stored in the activated hydrogen storagecomponent. Of course, a component storing hydrogen and a component thatdoes not store hydrogen therein may be used in combination.

The weight ratio of the activated hydrogen storage component to thehydrogen storage material may be 0.1% to 10% by weight.

The hydrogen storage material of the present embodiment may be producedby mixing and stirring the poorly activatable hydrogen storage componenttogether with the activated hydrogen storage component. If such aproduction method is used, the content of the activated hydrogen storagecomponent should be about 0.1% to 10% by weight.

If the activated hydrogen storage component is a substance that islikely to be deactivated in air, the activated hydrogen storagecomponent may be mixed with the poorly activatable hydrogen storagecomponent in an inert gas atmosphere such as nitrogen or argon.

The hydrogen storage material produced in the above manner may beactivated inside the hydrogen storage vessel. In this case, the hydrogenstorage vessel is heated to a predetermined temperature, and is filledwith hydrogen at a predetermined pressure. The temperature and thehydrogen pressure may be determined depending on the heat resistance andpressure resistance of the hydrogen storage vessel. Generally, thistemperature is 100° C. or lower and the hydrogen pressure is 10 MPa orless. More preferably, the temperature is 80° C., whereas the hydrogenpressure is about 4 to 8 MPa, respectively.

In accordance with the present embodiment, activation of the hydrogenstorage material is completed in a remarkably short period of time, ascompared to a case of activating a poorly activatable hydrogen storagecomponent on its own. For example, in the case of activating a poorlyactivatable BCC hydrogen storage alloy of V-13.5Cr-4Ti alone, at 80° C.and 5 Mpa respectively, the activation time for converting the alloy toa hydrogen storable state is long, requiring 75 hours. In thecomposition formula V-13.5Cr-4Ti, as well as in the composition formulaeto be described hereinafter, each numeral represents an atomicpercentage unless otherwise noted. In contrast, in the case of adding 1%by weight of an activated V-13.5Cr-4Ti alloy to an unactivatedV-13.5Cr-4Ti alloy, the activation time for converting the mixture to ahydrogen storable state is considerably shorter, requiring a time ofonly one hour. Also, in the case of using a mixture of another poorlyactivatable hydrogen storage component, the required activation timealso is shorter, requiring a time of at most 10 hours.

When the hydrogen storage material becomes deactivated in the hydrogenstorage vessel, the material can be activated again within a shortperiod of time. As described above, the hydrogen storage material can beactivated in a remarkably short time.

As made clear from the above, in the present embodiment, the hydrogenstorage material can be activated without complicated processes.Further, an additional element is not added during production of thehydrogen storage material, so that the hydrogen storage capacity perunit weight of the hydrogen storage alloy does not become deterioratedby such an additional element.

The reasons why the time required for activating the poorly activatablehydrogen storage component can be significantly shortened simply byadding the activated hydrogen storage component thereto are not fullyunderstood at present. However, it is presumed that a phenomenon, whichis similar to spillover in catalytic chemistry, is caused in thehydrogen storage material. Thus, it is presumed that an active hydrogenatom stored in the activated hydrogen storage component is transferredover to an oxide layer on the poorly activatable hydrogen storagecomponent, so that the oxide layer is reduced by the active hydrogenatom, whereupon the poorly activatable hydrogen storage componentbecomes activated into a hydrogen storable state.

EXAMPLE 1

According to the alloy composition formula V-13.5Cr-4Ti, 16.49 kg of V,2.75 kg of Cr, and 0.75 kg of Ti were melted in a high-frequency meltingfurnace under an inert atmosphere.

The obtained melt was cast to form an ingot. The ingot was mechanicallycrushed and classified, in order to obtain 15 kg of V-13.5Cr-4Ti alloyparticles, having an average particle diameter of 500 μm.

Then, 3 g of the particles were weighed, and placed inside a sealablevessel. The vessel was evacuated, then maintained at 400° C. at ahydrogen pressure of 8 MPa for one hour, and cooled to room temperature.The steps of evacuating, maintaining, and cooling the vessel wererepeated three times in order to obtain an activated V-13.5Cr-4Ti alloy.

0.1 g of the activated V-13.5Cr-4Ti alloy and 1.9 g of an unactivatedV-13.5Cr-4Ti alloy were added to a pressure-resistant vessel under anitrogen gas atmosphere. The pressure-resistant vessel was closed,whereupon the alloys were stirred and mixed to produce a hydrogenstorage material. Thus, the content of the activated V-13.5Cr-4Ti alloywas 5% by weight within the hydrogen storage material. In the samemanner, hydrogen storage materials having activated V-13.5Cr-4Ti alloycontents of 1% by weight and 3% by weight were produced at a yield of 2g respectively.

Each of the produced hydrogen storage materials was maintained for onehour in the pressure-resistant vessel at 80° C. and at a hydrogenpressure of 5 MPa. Then, the pressure-resistant vessel was cooled toroom temperature.

The hydrogenation rate of each of the hydrogen storage materials wasmeasured using a volumetric hydrogen pressure-composition isotherm (PCT)measurement apparatus. The hydrogen storage materials were introducedinto a sample cell of the PCT measurement apparatus, and the sample cellwas evacuated at 80° C. for one hour, then maintained at 80° C. and at ahydrogen pressure of 5 MPa for one hour, and cooled to room temperature.Measurements were initiated after completion of the cooling step.

For purposes of comparison, 2 g of an unactivated V-13.5Cr-4Ti alloy wasadded to a pressure-resistant vessel having the same shape, and thepressure-resistant vessel was maintained at 80° C. and at a hydrogenpressure of 5 MPa for one hour, after which the vessel was cooled toroom temperature. The hydrogenation rate of the alloy was measured inthe same manner as described above.

The results are shown in the graph of FIG. 1. In FIG. 1, elapsed time isshown on the horizontal axis, whereas the stored hydrogen amount of thehydrogen storage material is shown on the vertical axis. The storedhydrogen amount is represented by a weight ratio (% by weight) of thestored hydrogen to the hydrogen storage material.

As is clear from FIG. 1, the stored hydrogen amounts of the hydrogenstorage materials, which were produced by adding the activatedV-13.5Cr-4Ti alloy to the unactivated V-13.5Cr-4Ti alloy, at activatedalloy contents of 1%, 3% and 5% by weight respectively, increased overtime. This implies that the residual unactivated V-13.5Cr-4Ti alloy wasactivated, while the mixture was maintained at a temperature of 80° C.and a hydrogen pressure of 5 MPa for one hour.

On the other hand, in the Comparative Example using only the unactivatedV-13.5Cr-4Ti alloy, the stored hydrogen amount was not increased. Thus,the unactivated V-13.5Cr-4Ti alloy could not be activated by beingmaintained at a temperature of 80° C. and a hydrogen pressure of 5 MPafor one hour, without the addition of the activated V-13.5Cr-4Ti alloy.

Although not indicated in FIG. 1, the stored hydrogen amount of theunactivated V-13.5Cr-4Ti alloy was increased after maintaining the alloyunder a condition wherein the temperature thereof was 80° C. and thehydrogen pressure was 5 Mpa for 75 hours. Thus, the alloy was capable ofbeing activated after maintaining the above condition for 75 hours.

As is clear from the above results, the time required for activation canbe significantly shortened, simply by adding the activated V-13.5Cr-4Tialloy.

EXAMPLE 2

5 g of an Mg powder, 0.036 g of an Ni powder, and 0.023 g of an Fepowder were added, together with 18 stainless-steel balls having adiameter of 10 mm, in a ball mill pot having a volume of 80 ml. Hydrogenwas introduced into the ball mill pot at a pressure of 1 MPa, and thepot was closed.

Then, the ball mill pot was placed on a bedplate of a planetary ballmill having a diameter of 300 mm, and ball milling, as one type ofmechanical grinding, was carried out for 10 hours at a bedplaterevolution of 350 rpm and at a ball mill pot revolution of 800 rpm, inorder to obtain 5.059 g of MgH_(x) (0.1≦x≦2) doped with Ni and Fenanoparticles. Immediately after the ball milling, the surface of theMgH_(x) was in a hydrogenated state, and thus the MgH_(x) was in anactivated state. Further, the rate of reaction between the hydrogenatedsurface and oxygen was remarkably low, so that the MgH_(x) was notsignificantly deactivated in air.

0.1 g of MgH_(x) and 1.9 g of the above prepared unactivatedV-13.5Cr-4Ti alloy particles were added to a sealable pressure-resistantvessel, and then stirred and mixed in air to produce a hydrogen storagematerial. Within the hydrogen storage material, the content of theactivated MgH_(x) was 5% by weight. In the same manner, hydrogen storagematerials, each having an activated MgH_(x) content of 1% by weight and3% by weight respectively, were produced at a yield of 2 g.

Each of the produced hydrogen storage materials was activated in thesame manner as described above, by maintaining the materials at 80° C.and at a hydrogen pressure of 5 Mpa, for one hour inside thepressure-resistant vessel. Then, the pressure-resistant vessel wascooled to room temperature. Further, the hydrogenation rate of each ofthe hydrogen storage materials was measured using a PCT measurementapparatus.

The results are shown in the graph of FIG. 2, together with theaforementioned results of the Comparative Example. As clearly shown inFIG. 2, the time required for activation can be significantly shortenedalso in the case of adding the MgH_(x) activated hydrogen storagecomponent, which is derived from a substance other than the poorlyactivatable hydrogen storage component (the V-13.5Cr-4Ti alloy).

As described above, by mixing the activated hydrogen storage componentwith the poorly activatable hydrogen storage component in order toproduce the hydrogen storage material, the poorly activatable hydrogenstorage component can be activated in the hydrogen storage material in aremarkably short period of time.

Although certain preferred embodiments of the present invention havebeen shown and described in detail, it should be understood that variouschanges and modifications may be made therein without departing from thescope of the appended claims.

1. A hydrogen storage material comprising a poorly activatable hydrogenstorage component and an activated hydrogen storage component, wherein10 hours or more are required to activate said poorly activatablehydrogen storage component at a hydrogen pressure of 10 MPa or less andat a temperature of 100° C. or lower.
 2. A hydrogen storage materialaccording to claim 1, wherein said poorly activatable hydrogen storagecomponent comprises a body-centered cubic hydrogen storage alloy.
 3. Ahydrogen storage material according to claim 2, wherein saidbody-centered cubic hydrogen storage alloy is one of a V—Cr—Ti hydrogenstorage alloy, a V—Cr—Al hydrogen storage alloy, a V—Cr—Mo hydrogenstorage alloy, a V—Ti—Mo hydrogen storage alloy, a V—W hydrogen storagealloy, a V—Cr—Ti—Al hydrogen storage alloy, and a V—Cr—Mo—Al hydrogenstorage alloy.
 4. A hydrogen storage material according to claim 1,wherein said poorly activatable hydrogen storage component comprises oneof a Ti—Zr—Fe—Cr—Ni alloy, a Ti—Fe—Cr—Mn alloy, and a Ti—Fe—Cr—Cu alloy.5. A hydrogen storage material according to claim 1, wherein said poorlyactivatable hydrogen storage component comprises one of an La—Ce—Ni—Mnalloy and an La—Ce—Ni—Fe alloy.
 6. A hydrogen storage material accordingto claim 1, wherein said activated hydrogen storage component comprisesMgH_(x) (0.1≦x≦2) doped with a nanoparticle of at least one atomselected from the group of Ni, Fe, Ti, Mn, and V.
 7. A hydrogen storagematerial according to claim 1, wherein said activated hydrogen storagecomponent comprises an activated product of said poorly activatablehydrogen storage component.
 8. A hydrogen storage material according toclaim 1, wherein said activated hydrogen storage component comprisesactivated LaNi₅ or TiCr₂.
 9. A hydrogen storage material according toclaim 1, wherein a weight ratio of said activated hydrogen storagecomponent to said hydrogen storage material is 0.1% to 10% by weight.